Mechanical ventilation with low tidal volume (1-3 ml/kg) and high frequency (1-15 Hz) has been shown recently to significantly improve gas exchange in normal and abnormal subjects and animals. The mechanism responsible for this improvement is not known but the potential clinical application in acute respiratory distress syndrome (ARDS), chronic obstructive pulmonary disease (COPD) and other forms of lung disease is significant. This project proposes an engineering development and evaluation of a dual mode wide frequency band positive pressure ventilator. The performance characteristics of this ventilator will encompass the ventilation spectrum between the present two modes of high frequency ventilation. The ventilator design is based on high frequency response valves manufactured by Carleton Controls Corporation. This design has the capability for 0.1 to 30 Hz frequencies, 2:1 to 1:5 inspired-to-expired time ratios and up to 175 ml pulse volumes. A second design with promising preliminary results, based on flow interruption by a rotating disc, is also proposed. The mechanical performance of the ventilator will be tested on a physical lung model. The ventilator will then be evaluated in animals to determine settings for optimal gas exchange. Arterial blood gas measurements will be used to determine relative maximum PaO2 values over the range of ventilator settings. The multiple inert gas and 133-Xenon radionuclide methods will then be used to characterize gas exchange at these optimal ventilator settings. Normal anesthetized and paralyzed dogs and dogs with oleic acid-induced hemorrhagic pulmonary edema will be used as the normal and pathologic animal models.